Prevent iteration over empty polygon

When utilizing the standard iterator container, we may have polygons
with an empty outline (these are generated by gerbview to pcbnew).  This
should register as the end of the iterator to prevent dereferencing

include/geometry/shape_poly_set.h

@@ -165,7 +165,9 @@ class SHAPE_POLY_SET : public SHAPE
 
             operator bool() const
             {
-                return m_currentPolygon <= m_lastPolygon;
+                return ( ( m_currentPolygon < m_lastPolygon ) ||
+                        ( m_currentPolygon == m_lastPolygon &&
+                          m_currentVertex < m_poly->CPolygon( m_currentPolygon )[0].PointCount() ) );
             }
 
             /**